While figuring out what we wanted to build, I used Google Sketchup (the free version) to create a 3D model of the house and garage. Working with 3D takes a bit of getting used to, but it's very similar to the solid modeling I was doing to make custom 3D printed parts for my homebrew phone, and Sketchup is very good at getting these kind of artists-impression images created.
The house really is a box, and the full width deck with steps going back down the side doesn't exist yet, but I was able to get the relative position and size of the garage about right so we could see if it fitted in. We then went to TuffShed and got them to draw up plans for our oversized garage. If you get a garage custom "stick built" at the site, the cost is quite high, mostly labor. Tuffshed pre-fabricate the parts, ship them in, and put up the entire building in about three days. The cost (not including foundations) is about $30K for an 1150 square foot triple wide garage (32ft wide, 36ft deep), with a 16ft wide double door, 8ft wide single door, and 10ft high ceilings. The basic cost is more like $20K, but it cost extra to go from 8ft to 10ft ceilings, and we added a bunch of other extra options.
So far, we have had a few minor issues getting the Tuffshed plans approved by the county planning department. Really just clarifications of materials and codes on the drawings, but the first two tries have failed. Everything is now approved apart from the final tweaks to the drawings of the garage, so I'm hoping to get it taken care of next week.